Abstract

Simultaneous observations have been made of an auroral arc by an all-sky TV imager and of plasma flows near the arc by the Scandinavian Twin Auroral Radar Experiment. After the Harang Discontinuity, during a quiet geomagnetic interval (Kp = 1 +), the arc undergoes a sudden brightening during an eastward surge. This is accompanied by a vortex in the plasma drift velocities, poleward of the arc, of spatial extent 180 × 140 km. Westward drifts at higher latitudes merge with eastward drifts at lower latitudes near the arc. No radar backscatter was recorded equatorward of the arc. The plasma flow vortex is shown to correspond with an ionospheric region of diverging horizontal electric fields, which is equivalent to a downward field-aligned current. This region may correspond to black auroras. Spatial location of the closure current region associated with an arc by ground-based observations is a novel result.